The present invention relates to a crucible for use in the pendant drop crystal growth method for preparing single crystals, and more particularly, it involves improvements in the crucibles heretofore used for such methods.
Crucibles for use in the pendant drop growth (PDG) method for preparing single crystals have been described in French Pat. Nos. 2,321,326; 2,359,639, and 2,376,697. These crucibles have been utilized particularly for the preparation of single crystals in the form of fibers or plaques (see French Pat. No. 2,321,326), for the preparation of single crystals in the form of tubes (see French Pat. No. 2,359,639) and for the deposition of thin films of crystalline silicon on graphite substrates (see French Pat. No. 2,401,696).
In the crystallization procedure called the pendant drop growth method, also known as the PDG method, there is utilized a crucible having in its lower region a capillary bore. The crucible fulfills two functions: First, it serves to melt the material (the powder or crushed crystal) to be crystallized by an appropriate heating means, such as by a resistive heater or by direct induction heating in the crucible; second, by virtue of the capillary bore provided in the lower region, the crucible gives to the liquid initially passing therethrough the form of a drop at the lower edge of the capillary, the drop then hanging at that lower edge.
When this capillary channel is filled and the drop has formed, a seed is advanced to the molten drop at the lower end of the capillary and is touched to the drop. There is thus established a liquid-solid interface. Commencing at that time, the seed is drawn downwardly away and provides for the growth of a crystal, the liquid-solid interface being maintained at a suitable level through the use of an appropriate temperature gradient. At the same time, powder or crushed crystal is fed to the upper part of the crucible, where the melting of the feed material is carried out.
Under these conditions of making single crystals by the pendant drop growth method, it has been found that numerous phenomena can lead to defects in the single crystal so pulled.
(a) The capillary orifice situated in the base of the crucible is a rather narrow mouth through which the liquid is constrained to pass. This opening for the liquid, which experimentally comprises a thin layer of 0.5 to 1 mm or more at the bottom of the crucible, causes irregular liquid flow both across the length of the orifice and in the manner of entering the orifice.
In effect, the feed powder falls down chiefly at the center of the crucible and it melts in place. The liquid obtained entirely fills the bottom of the crucible. Nevertheless, the center of the crucible is better fed with still unmelted powder than the outer portions, which results in the central part of the orifice being filled in a more regular fashion than the edges. Some temperature differences between the center and the edges of the crucible have an effect on the viscosity and the surface tension of the liquid, which also causes some local differences in the ability of the liquid to enter the capillary orifice.
(b) On the other hand, the capillary orifice possesses a sharp ridged edge, which produces turbulent flow of the liquid at that spot, causing zones of expansion-compression in the interior of the channel. Since the process operates above or very close to the melting point of the liquid, this provokes cavitation phenomena, that is, local vaporization of the liquid, and thereby causes bubbles in the liquid in the interior of the capillary channel. These bubbles can be detected in the crystal where they create voids of various forms, such as elongated, cylindrical, or spherical.
The phenomena described above also disturb the temperature equilibrium below the capillary channel at the level of the liquid-solid growth interface, and this tends to cause various defects, such as unmelted beads and dislocations in the crystal.
Moreover, this thin film of liquid is cooled by radiation toward the top of the crucible. To the extent that the liquid layer does not have exactly the same thickness over the entire bottom of the crucible, this acts to increase temperature differences across the liquid which enters the entire width of the capillary opening.
Additionally, the quantity of liquid present in the channel of the crucible is small, on the order of 0.5 cm.sup.3 for a crucible serving to pull sapphire ribbons 30 mm across and 0.8 mm thick. Thus, if the amount of powder fed varies, this quantity of liquid varies and equally the feed to the channel varies, which may cause variations of crystal thickness.